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"Progressive" for JPEGs means it's the sort that opens in several passes. It's designed for modems (the extra-slow sort, eg. 2400 baud), and it looks terrible (everything is blurry/blocky until it finishes loading).

(The rough PNG equivalent of this is called "interlaced". Don't use that, either. Don't get these mixed up with video, where "progressive" and "interlaced" are opposites.)